Im not promoting the 99s, saying I buy into the marketing, or trying to imply anyone notable has tried it or will try it.

I did watch one of the Wilson promo videos for the 99s and the new "spin effect" technology.

Their explanation was that polyester strings move more freely and snap back into position after making impact with the ball. That is why players dont have to "adjust" their poly strings in between points as much when compared to non polyester string. Ball impact makes all string move but non polyesters dont move back into their original positions like polyesters do which ends up making the strings be out of position after rallies.

So Wilsons thought was making a less dense string pattern and using a polyester string would allow the string to move even more. Since the string could move more it would impart more spin on the ball because it snapped back more.

You can go watch Wilsons video explaining it all its too much for me to type here.
Again not saying I am promoting it, I am just explaining Wilsons reasoning for it.